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PICList Thread
'Electrocardiogram (PIC ECG)'
1999\03\27@135515 by Thomas McGahee

As wonderful as the PIC is, it is not always the best solution for
*all* problems. Sometimes an op amp or some other piece of hardware
greatly simplifies the solution.

An OpAmp amplifier followed by a 60 Hz active reject filter would be an
appropriate solution.

If you are interested in only signals *below* a certain frequency,
then a low pass filter might also be added.

If you are interested only in a very narrow band of frequencies,
then a bandpass filter would do the job.

Don Lancaster's "Active Filter Cookbook" has a decent treatment
of the kinds of circuits that might be of interest to you.

The use of modern single-supply OpAmps would eliminate the
need for a negative power supply in most designs.

Fr. Tom McGahee

{Quote hidden}

1999\03\27@191931 by Marcos Migliorini

Tom you are rigth but think about an EEG machine...... You need a lot of
filters and also you need min 8 channels!; some EEG machines have 24
channels. In this case is very useful use a DSP in orther to perform the
digital filter, may be one DSP/channel.
What do you think?
{Original Message removed}

1999\03\28@052714 by paulb

Thomas McGahee wrote:

> As wonderful as the PIC is, it is not always the best solution for
> *all* problems. Sometimes an op amp or some other piece of hardware
> greatly simplifies the solution.

 I'd certainly tend to wonder here too, how a PIC became involved in a
discussion of ECG signals?

 On second thoughts however, it may not be an unreasonable question.
The almost-8-bit accuracy of a PIC with ADC is probably sufficient for
ECG monitoring, and with the sampling rate limited to say, 600/s a basic
"switched capacitor" synchronous filter can be implemented with only
five two-byte "bins".

 Each of these performs a FIR filter function with a little wrinkle -
the signal to the filter is inverted every 1/120 second and the output
(inverted as appropriate) of the filter is subtracted from the current
sample before passing it on to the next stage of processing.

 This crude DSP implements a very deep and very narrow notch at the
mains frequency (needs to be switchable for country).

 Presumably the next stage in the processing would be to serialise the
datastream and send it through a fairly basic optocoupler for isolation.
A PIC is on consideration, not a bad choice for this.  You have four
analog inputs and multiplexer already, the PIC can choose on the basis
of a(n opto-isolated) command signal or just perform the above
processing on all four.  Good enough for a monitor.

 The inputs from the leads would have something of the order of 2 to 5
megohm high-quality isolating resistors.  I believe there is also a
device available with negative resistance function (a bit like a PTC
thermistor) to block more than a few microamps of input current for this
purpose also.

 Finally, the PIC can implement a smart "baseline restore" function in
conjunction with a 4066-type device on the op-amp.  A "sample-and-hold"
in reverse.  Whenever the signal limits for more than a predetermined
time, the S&H is activated to "zero" (mid-range) the op-amp output.

 Interesting application.
       Paul B.

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